bluetooth: hci_core: handle NUM_COMPLETED_PACKETS rsp

The `BT_HCI_OP_HOST_NUM_COMPLETED_PACKETS` is not expected to generate a
response from the controller, but from the spec:

Normally, no event is generated after the
HCI_Host_Number_Of_Completed_Packets command has completed. However, if
the HCI_Host_Number_Of_Completed_Packets command contains one or more
invalid parameters, the Controller shall return an HCI_Command_Complete
event with a failure status indicating the Invalid HCI Command
Parameters error code.

In practice, this can also be generated if flow control is inadvertedly
turned off during operation. Running `hci_cmd_done` is not correct when
the event happens (as there is no corresponding command buffer), and
`ncmd` should not be returned as this command ignores the semaphore when
sending.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>